1. Field of the Invention
The present invention relates to an image forming optical system being suitable for use with an image pickup apparatus.
2. Description of the Related Art
The equipping of automobiles with vehicular mounted cameras has begun to spread. At first, vehicular mounted cameras were employed mainly as monitors to provide visual confirmation. Recently, however, vehicular mounted cameras have begun to be employed as high performance sensing cameras, used, for example, for detecting white lines on roads, for performing monitoring to prevent cross-lane veering, for detecting obstacles and for observing drivers and their performance. As a result, there is a growing demand for a reduced size vehicular mounted camera, manufactured at low cost, that provides a wide angle of view, i.e., which completely eliminates blind spots, by employing a fisheye optical system, for which only a small number of lenses is required, that has a half angle of view equal to or greater than 90° and a high image quality.
Examples of the fisheye optical system, constituted by a small number of lenses, are described in Japanese Patent No. 2,992,547 and Japanese Patent Laid-Open Application No. 2002-72,085. Six lenses are employed for each of these optical systems. Other examples of a wide-angle optical system composed of six lenses are described in Japanese Patent Laid-Open Application No. Hei 5-264,897, No. 2000-352,665, No. 2001-281,540 and No. 2004-317,866.
Further, an example of an optical system composed of seven lenses is described in example 12 in Japanese Patent Laid-Open Application No. 2000-352,665.
Furthermore, examples of fisheye optical system having a half angle of view exceeding 90° are described in Japanese Patent Publications (Kokoku) No. Sho 51-14,017 and No. Sho 51-2,826.